Storms and vortices

The atmosphere of Neptune is observed to be highly dynamic and convective, with numerous storm and eddy features that allowed early estimates of the planet's rotation rate to be made due to diurnal variations of the planet's observed reflectivity. As telescope technology improved, ground-based images in the 0.89 ^m methane band began to resolve distinct high-altitude cloud features on scales of 10,000 km or more (Baines, 1997b) and the distribution of these features was found to be highly variable.

The arrival of Voyager 2 at Neptune in 1989 heralded an enormous development in our understanding of clouds and storm systems in Neptune's atmosphere (Smith et al., 1989). Voyager 2 observed four large features that persisted for the duration of the Voyager observations (from January to August 1989). The largest of these was the Great Dark Spot (GDS) and its white companion immediately to the south, which together drifted from 26°S to 17°S during the period of observation, or equivalently

Figure 5.39. Detail of Neptune's GDS and DS2 observed by Voyager 2 in 1989. Courtesy: NASA.

drifted equatorwards at a rate of 15° yr-1 (Figure 5.39). Such a drift of these features down the potential vorticity gradient towards the equator is expected from dynamical modeling (LeBeau and Dowling, 1998). Furthermore, once such features get too close to the equator, they are expected to rapidly dissipate through conversion into planetary waves (Sromovsky et al., 2001c), as was discussed in Section 5.3.3. At first glance, the GDS appeared similar to the Great Red Spot of Jupiter since they both rotated anticyclonically and were of comparable size. However, in other respects they were very different. For a start the GDS appears to have been a short-lived disturbance, which by the time of later HST observations in 1994 had completely disappeared. Second, the GRS is believed to be a region of rapid updraft and the cloud cover is particularly thick and high, whereas the dark color of the GDS may be attributed either to a low methane ice and stratospheric haze abundance above the GDS or, more likely, suggests that that the main 3.8 bar cloud itself is somehow darker, or deeper. The wispy white clouds associated with the companion to the GDS were observed to move at a different speed to the GDS and companion, suggesting that these features form and evaporate high above the GDS as they pass through a local pressure anomaly (West, 1999). Stratman et al. (2001) performed dynamical modeling that supported this view and further determined that the cloud tops must be just below the tropopause. The second dark spot (DS2) was observed at 55°S in the dark circumpolar band and appeared to be roughly 180° in longitude away from the transient bright clouds seen at 70°S, called the South Polar Feature (SPF) (Sromovsky et al., 1993). An additional bright cloud was seen near 42°S and acquired the name "Scooter".

A striking feature of Neptune's dark spots, discovered by Voyager 2, was that they oscillated. The longitudinal and latitudinal widths of the GDS were found to vary sinusoidally and in antiphase, with a period of approximately 200 hours and with amplitudes of 7.4° longitude and 1.5° latitude (Sromovsky et al., 1993). While the shape of DS2 did not undergo such oscillations, its position was found to vary sinusoidally with a period of 36 days and amplitude of 2.4° latitude and 47.5° longitude (relative to longitudinal mean drift). These positional variations were accompanied by a variation in the area of the bright core of the DS2 which had maximum area when the DS2 was farthest north.

After the Voyager 2 encounter, high-resolution imaging of Neptune did not begin again until HST observed the planet in 1994. Remarkably it was discovered that all the discrete atmospheric features observed by Voyager 2, with the exception of the SPF, had completely disappeared (Hammel et al., 1995)! In their place a new Great Dark Spot had formed in the northern hemisphere at 32°N, which acquired the name NGDS-32 (Figure 5.40). Unlike the Voyager GDS, NGDS-32 did not drift equatorwards (Sromovsky et al., 2002), but was seen at the same latitude in 1996 HST observations (Sromovsky et al., 2001c), together with a new dark spot at 15°N (NGDS-15). However, NGDS-15 had completely disappeared by the next HST observations in 1998 (Sromovsky et al., 2001d) and only the bright companions of NGDS-32 were still visible. The remains of NGDS-32 have now completely disappeared and no further dark spots have since been seen on Neptune.

In addition to ongoing HST observations, advances in the deconvolution of telescope images (Sromovsky et al., 2001a) and the development of adaptive optics have greatly improved the spatial resolution of ground-based observations at visible and near-IR wavelengths, and have thus greatly improved the time sampling and spectral sampling of Neptunian clouds. Numerous clouds have now been recorded in Neptune's atmosphere over a number of years (Figures 5.41 and 5.42, see color section for both). Transient clouds generally appear between 29°S and 45°S, and 29°N and 39°N [NB: 39°N is the farthest north that can currently be seen due to Neptune's obliquity and season] and convective activity has been seen to increase

Figure 5.40. HST image of a new "Great Dark Spot'' on Neptune, NGDS-32, located at 32°N, recorded in 1994. By this time the Voyager GDS found in the southern hemisphere had completely disappeared. Courtesy of NASA.

steadily since the 1990s, which seems to account for the steady rise in Neptune's disk-averaged visible albedo since the start of continuous measurements in 1950 (Lockwood and Jerzykiewicz, 2006). These latitudes correspond to the cooler tropo-pause temperatures indicative of upwelling and divergence at high altitudes discussed earlier.

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